Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680X0 processor family, manufactured by Motorola, Inc. of Phoenix, Ariz.; the Intel 80X86 processor family, manufactured by Intel Corporation of Sunnyvale, Calif.; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, Calif. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.
Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor's instruction set. As an example, the instruction set used by the Intel 80X86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80X86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a higher rate.
The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80X86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system's hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.
A computer manufacturer will want to maximize its functionality by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturer's product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will run an emulator program that allows the host computer to emulate receiving and executing the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. In some cases, the host computer can both run software designed for its own hardware architecture, other than the emulation program, and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and programs written for PC-based computer systems. It may also be possible to use an emulator program to concurrently operate multiple incompatible operating systems on a single CPU. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.
When a guest computer system is emulated on a host computer system, the guest computer system is said to be a virtual machine, as the guest computer system exists only as a software representation of the operation of the hardware architecture of the guest computer system. The terms emulator and virtual machine are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, Calif. emulates an entire computer that includes an Intel 80X86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system. The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment.
One advantage of a virtual machine over a real machine is the ability to quickly and cheaply create multiple instances of virtual machines. If allowed by the virtual machine implementation, multiple virtual machines can exist simultaneously in a single host machine (host computer system) environment. Resources of the host machine can be divided among the various virtual machines. For example, a single host machine with four processors and 1 gigabyte of random access memory (RAM) could be divided evenly into four virtual machines, each of which is given one processor and 256 megabytes of RAM. Other resource allocation divisions are possible.
This flexible resource allocation becomes even more useful when combined with the ability to move virtual machines from one host machine to another. This allows for “load balancing” of systems. For example, if a virtual machine requires more processing power than is available on one host machine, it can be moved to another host machine that has extra capacity.
In some computing environments, it is useful to have multiple machines that are nearly identical in configuration (both hardware and software). For example, a large electronic-commerce web site such as Amazon.com has dozens or hundreds of web servers that are all nearly identical in configuration. This setup allows for easy expandability. When current capacity is inadequate, additional servers can quickly be brought on line.
Another case where nearly-identical configurations are useful is in the testing of configuration modifications. When dealing with mission-critical applications, IS managers often want to test software configuration changes before applying them to the production system. For example, if a new “security patch” was made available by Microsoft Corporation for the Windows operating system, an administrator may want to test this patch on a separate server machine before installing the patch on the production server.